The magnetic recording media essentially comprise a support such as a polyester film having provided thereon a coated film (that is, a magnetic recording layer, which is hereinafter referred to as a magnetic layer) comprising ferromagnetic fine particles (such as .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, their modified iron oxide, or CrO.sub.2) and at least one thermoplastic resin such as a resin comprised of a vinyl chloride/vinyl acetate copolymer, a resin comprised of a vinyl chloride/vinylidene chloride copolymer, a cellulose type resin, an acetal resin, a urethane resin, an acrylonitrile butadiene copolymer type resin or the like.
Acicular fine particles of various ferromagnetic particles are used as the ferromagnetic fine particles in this field of art.
Recently, there has been a great demand for video tapes and audio tapes having higher efficiencies, higher recording densities, and higher reproduced outputs have been strongly desired. In order to achieve these objects, there has been a tendency to reduce the size of the ferromagnetic particles present in these magnetic tapes. Because of the reduction of the size of the ferromagnetic particles and the high magnetic moment of the particles themselves, the particles have a tendency to coagulate. As a result thereof, it has been difficult to homogeneously disperse the particles in a binder.
In order to solve the above technical problems, it has been proposed to improve the affinity of a binder for ferromagnetic particles, the dispersibility of ferromagnetic particles in the binder, and the signal/noise (S/N) ratio of the thus formed magnetic layer by introducing a PO(OM).sub.2 group, an OPO(OM).sub.2 group, an OP(OM).sub.2 group, a SO.sub.3 M group, an OSO.sub.3 M group, a COOH group and an OH group (in which M represents a hydrogen atom, an alkyl group, an alkali methal atom, an ammonium group, or an amino group) into the molecular chain of the above described resins as binder components (as described, e.g., in Japanese Patent Application (OPI) Nos. 59623/86, 172213/86, 133012/86, 133013/86, 172213/86, and 177524/83) (the term "OPI" used herein means a published unexamined Japanese patent application).
On the other hand, in order to improve dispersibility of ferromagnetic particles and to smooth the surface of a magnetic layer, it is proposed that: ferromagnetic particles are treated with silicon oil; a silane coupling agent capable of reacting with the binder resin is added to the binder; methods using aminosilane compounds or Si compounds; methods using lubricating agent; and ferromagnetic particles are treated with oleic acid. These methods are described, e.g., in U.S. Pat. Nos. 4,309,459, 4,323,464, 4,336,310, 4,332,863, Japanese Patent Application (OPI) Nos. 186302/82, 155517/83, 224102/84, 140527/81, 155703/83, and Japanese Patent Publication No. 1048/82.
However, in the magnetic layer using the above described resins as binders, the dispersibility of the ferromagnetic particles is still poor. Therefore, there are problems in that the surface properties of the magnetic layer are poor, the residual magnetic flux density and the squareness ratio are unsatisfactory, and the magnetic layer is ready to be damaged and powders readily come off, resulting in staining parts of tape running system of a magnetic recording medium while the medium runs.
Further, there is also a tendency for drop outs to increase due to dust and contaminants which are formed by scraping the magnetic layer, backing layer, or support, and stains in the tape running system.